Studies were conducted to determine the requirements for immunoglobulin and complement for opsonization of Bacteroides fragilis and Bacteroides thetaiotaomicron. The ability of human sera depleted of immunoglobulin or complement components to promote phagocytosis and intracellular killing of the Bacteroides strains by human leukocytes was measured in vitro under anaerobic conditions. Neither hypogammaglobulinemic sera nor heated pooled normal human serum (PNHS) (56 degrees C, 30 minutes) supported phagocytosis and killing of the Bacteroides strains. Sera depleted of terminal complement components by treatment with insulin or cobra venom factor and C8 deficient human serum did not support phagocytosis of the test strains. PNHS depleted of C3, factors B or D also did not support phagocytosis of either strain. Dose-dependent restoration of the opsonic activity of factor B depleted serum was accomplished by purified human factor B, but not by human C2. The results indicated that immunoglobulin and components of the alternative complement pathway participate in opsonization of the Bacteroides strains tested in this study.